Optical fibers used for the transmission of light signals have a cylindrical structure in which the index of refraction of the central region of such structure is greater than the index of refraction of the periphery. It is conventional to refer to such central region as the core and to the periphery as the sheath or cladding. The numerical aperture of such a fiber increases with increasing divergence between the indices of refraction of the core and cladding.
It is known in this field that the best optical fibers are made of silica. It is also known that the index of refraction of silica can be modified by addition of doping elements, with titanium, aluminum or germanium oxides increasing the index of refraction and boron and fluorine decreasing it. Thus optical fibers with titanium-doped cores or fluorine-doped claddings are known in the art.
British Pat. No. 1,492,920, for example, describes a process of preparing vitreous silica, doped with fluorine and free of OH ions, by reaction of a silicon compound, such as SiCl.sub.4, and a fluorine compound, with the oxygen contained in a hydrogen-free gaseous current in the flame of an induction plasma burner. The compound used to dope the silica is a fluorocarbon compound, namely, dichlorodifluoroemethane, CCl.sub.2 F.sub.2, added in vapor form to the oxygen introduced in the plasma burner. This compound decomposes in the very hot flame of the plasma at the same time that SiO.sub.2 is formed. The vitreous silica, thus doped with fluorine, is deposited radially on the surface of a cylindrical blank of pure silica or one doped with metal oxides. See also British Pat. Nos. 1,391,177 and 1,431,352 and Canadian Pat. No. 1,029,993. See also applicants' U.S. patent application Ser. No. 233,512 which is not part of the prior art. application Ser. No. 233,512 is incorporated herein by reference.
Advantageously, optical fibers should have the lowest possible transmission losses from absorption and diffusion. Such low losses, however, are typically achievable over only a limited spectral range. Thus, those types of optical fibers that are transparent in the infra-red region are not transparent in the visible region and vice versa.
One cause of light attenuation in optical fibers is structural defects. Such defects are typically caused during formation of the fiber, during heat treatment or during irradiation. Structural defects can be avoided by incorporating considerable quantities of hydroxyl groups into the silica. While this makes it possible to obtain optical fibers having low attenuation at wavelengths near 630 nanometer (nm), the hydroxyl ions produce intense absorption bands in the near infrared at 950 nm and longer wavelengths. Conversely, if the concentration of hydroxyl ions in the optical fiber is low (less than about 5 parts per million (ppm)) the optical fiber typically is not usable at wavelengths less than 640 nm because of light absorption caused by structural defects.
Further information concerning light absorption in optical fiber may be found in: Kaiser, et al., Journal of the Optical Society of America, Vol. 63, No. 9, p.1141 (1973); Friebele, et al., Second European Symposium on Optical Fibers (Paris 1976); Friebele, et al., Appl. Phys. Lett., Vol. 24, p. 412 (1974).